A wireless communication system including a base station and a mobile station is known (see, for example, Japanese Laid-open Patent Publication Nos. 2004-297444, 2003-110460, and 2013-240102, Li Baojin and 2 others, “Interference Cancellation for HetNet Deployment in 3GPP LTE-Advanced Rel-11”, Proceedings of IEEE Vehicular Technology Conference, IEEE, pages 1 to 5, June 2013, and Henao J. C. and 15 others, “Advanced Receiver Signal Processing Techniques: Evaluation and Characterization”, Advanced Radio Interface Technologies for 4G Systems (retrieved from <URL: https://ict-artist4g.eu/projet/work-packages/wp2/deliverables/d2.2/final/d2.2-1/at_download/file> on the Internet on Jul. 15, 2014). The mobile station includes a receiving device that receives a radio signal from the base station. Further, the base station includes a receiving device that receives a radio signal from the mobile station. The mobile station performs radio communication with the base station in a radio area formed by the base station.
For example, in Release 10 of LTE specified by 3GPP, inter-cell interference control described as eICIC is proposed. 3GPP is an abbreviation for Third Generation Partnership Project. LTE is an abbreviation for Long Term Evolution. Further, eICIC is an abbreviation for enhanced inter-cell interference coordination.
As illustrated in FIG. 1, for example, the inter-cell interference control is assumed to be executed when a pico cell C2 is formed by a pico base station 92 inside a macro cell C1 formed by a macro base station 91. In this case, there is likely to be an increase in interference due to a radio signal transmitted in the macro cell C1 for a radio signal received from the pico base station 92 by a mobile station 93 located inside the pico cell C2. Hereinafter, the radio signal received from the pico base station 92 by the mobile station 93 located inside the pico cell C2 will also be described as a desired signal. Further, the radio signal transmitted in the macro cell C1 will also be described as an undesired signal or interference waves.
In the inter-cell interference control, therefore, a period P1 for transmitting an ABS in the macro cell C1 is provided, and data is transmitted to the mobile station 93 in the pico cell C2 during the period P1, as illustrated in FIG. 2. ABS is an abbreviation for almost blank subframe.
The ABS includes a reference signal (CRS, for example), a synchronization signal (PSS and SSS, for example), and a broadcast signal. CRS is an abbreviation for cell-specific reference signal. PSS is an abbreviation for primary synchronization signal. SSS is an abbreviation for secondary synchronization signal. The broadcast signal is transmitted via a physical broadcast channel (PBCH), for example. The ABS is a subframe in which a radio resource is allocated only to the reference signal, the synchronization signal, and the broadcast signal.
According to this configuration, the influence of the interference waves on the reception quality of the mobile station 93 is reduced in the period P1. Consequently, the reception quality of the mobile station 93 is enhanced. Accordingly, the area of coverage of the mobile station 93 by the pico cell C2 is expanded. The expansion of this area is also described as cell range expansion (CRE).
Meanwhile, the reference signal, the synchronization signal, and the broadcast signal included in the ABS degrade the reception quality of the mobile station 93. In Release 11 of LTE specified by 3GPP, therefore, feICIC is proposed. Herein, feICIC is an abbreviation for further enhanced ICIC.
In feICIC, information concerning an interference cell is transmitted to a mobile station. The interference cell is a cell in which a radio signal causing interference waves is transmitted. The information concerning the interference cell is also described as CRS assistance information. Further, in feICIC, the reference signal, the synchronization signal, and the broadcast signal included in the ABS transmitted in a macro cell are cancelled in a reception process by the mobile station. Thereby, the reception quality of the mobile station is enhanced. Accordingly, the area of coverage of the mobile station by a pico cell is expanded.
A receiving device that cancels the reference signal, the synchronization signal, and the broadcast signal included in the ABS transmitted in the macro cell is also described as an interference cancelling (IC) receiving device. Cancellation of the reference signal, cancellation of the synchronization signal, and cancellation of the broadcast signal are described as CRS-IC, PSS/SSS-IC, and PBCH-IC, respectively.
An overview of an IC technique will be described as an example. A received signal in a resource element (RE) in the k-th subcarrier and the l-th symbol time in OFDM is expressed by mathematical formula 1 described below. OFDM is an abbreviation for orthogonal frequency-division multiplexing. The l-th symbol time is a time corresponding to the l-th OFDM symbol along the time axis.
                              y          ⁡                      (                          k              ,              l                        )                          =                                            H              ⁡                              (                                  k                  ,                  l                                )                                      ⁢                          V              ⁡                              (                                  k                  ,                  l                                )                                      ⁢                          x              ⁡                              (                                  k                  ,                  l                                )                                              +                                    ∑                              i                =                1                                            N                cell                                      ⁢                                                  ⁢                                                            H                  i                                ⁡                                  (                                      k                    ,                    l                                    )                                            ⁢                                                x                  i                                ⁡                                  (                                      k                    ,                    l                                    )                                                              +                      n            ⁡                          (                              k                ,                l                            )                                                          (        1        )            
Herein, y(k, l) represents the received signal, and corresponds to an Nr×1 vector. Nr represents the number of receiving antennas used by the mobile station. H(k, l) represents a channel between the base station and the mobile station in a serving cell, and corresponds to an Nr×Nt matrix. The serving cell is a radio area formed by the base station and providing a radio resource used by the mobile station to transmit and receive data to and from the base station. Nt represents the number of transmitting antennas used by the base station in the serving cell.
V(k, l) represents an Nt×Nstream transmitting precoding matrix in the serving cell. Nstream represents the number of streams included in a desired signal. The desired signal is a radio signal transmitted to the mobile station by the base station in the serving cell. The desired signal is also described as a transmitted signal. Further, x(k, l) represents the desired signal, and corresponds to an Nstream×1 vector.
Ncell represents the number of interference cells. The interference cells are radio areas different from the serving cell. Hi(k, l) represents a channel between the base station and the mobile station in the i-th interference cell, and corresponds to an Nr×Nt,i matrix. Nt,i represents the number of transmitting antennas used by the base station in the i-th interference cell.
Further, xi(k, l) represents an undesired signal transmitted in the i-th interference cell, and corresponds to an Nt,i×1 vector. The undesired signal is the reference signal, the synchronization signal, or the broadcast signal, for example. Further, n(k, l) represents AWGN, and corresponds to an Nr×1 vector. AWGN is an abbreviation for additive white Gaussian noise.
The mobile station estimates a channel He,i between the mobile station and the base station in the i-th interference cell. The channel between the mobile station and the base station in the interference cell is also described as an interference channel. As indicated in mathematical formula 2 described below, the mobile station subtracts a signal obtained by multiplying the undesired signal xi by the estimated interference channel He,i from the received signal y, to thereby cancel a component of the received signal attributed to the undesired signal transmitted in the interference cell. The signal obtained by multiplying the undesired signal xi by the estimated interference channel He,i is also described as a replica signal of the undesired signal.
                                          y            pc                    ⁡                      (                          k              ,              l                        )                          =                              y            ⁡                          (                              k                ,                l                            )                                -                                    ∑                              i                =                1                                            N                cell                                      ⁢                                                  ⁢                                                            H                                      e                    ,                    i                                                  ⁡                                  (                                      k                    ,                    l                                    )                                            ⁢                                                x                  i                                ⁡                                  (                                      k                    ,                    l                                    )                                                                                        (        2        )            
Herein, yPC represents the signal obtained by cancelling the component of the received signal attributed to the undesired signal transmitted in the interference cell. The mobile station performs a reception process (such as estimation of the channel in the serving cell, demodulation of the received signal, and error correction decoding on the received signal, for example) based on the received signal yPC subjected to the cancellation.